Twenty years ago this week, a jury acquitted the Los Angeles policemen who beat motorist Rodney King, and the city exploded in a six-day riot. Before it was over, there were more than 50 deaths, about 2,500 injuries, and half a billion dollars or more in property damage. The riot led to alarming predictions that a new age of urban unrest might be at hand. What happened in the next two decades, though, was very nearly the opposite: Cities in the United States have been relatively riot-free over the last two decades.